Two-Way Radio Tutorial
The Antenna Farm: Your Two-Way Radio Source
Welcome to our Two-Way Radio Tutorial. We’ll try to keep it quick and as close to a “no brainer” as possible.
Two-Way 101
A two-way radio is a radio that can both transmit and receive (a transceiver). A two-way radio (transceiver) allows the operator to have a conversation with other similar radios operating on the same radio frequency (channel). Two-way radios are available in mobile, base and hand-held portable configurations. Two-way radio systems usually operate in half-duplex mode. The operator can talk or listen, but not at the same time (in other words, you push the button to talk and release it to listen). A push-to-talk (or press-to-transmit) button activates the transmitter. When it’s released, the receiver is active. A mobile phone or cellular telephone is an example of a two-way radio that both transmits and receives at the same time (called full-duplex mode). It uses two different radio frequencies (channels) to carry the two directions of the conversation simultaneously.
OK, now that we have the boring technical jargon out of the way, let’s get to the meat of the matter: “which radio do I need?” We’re glad you asked. Let’s go over the fundamentals and then look at some examples of two-way radios.
Before choosing a two-way radio, consider your needs.
First, are you buying a radio to join an existing talk group? If so, the first thing you need to know is which frequency (or frequencies) that talk group uses. If you don’t have an existing talk group and you’re starting a new one, then you need to ask yourself: “do I want VHF or UHF radios?”
NOTE: Before we go any further, please understand this. It is NOT against the law to purchase any two-way radio equipment. You do NOT need a license of any kind, or any written authorization, to purchase ANY two-way radio equipment we offer on our website. You will still need to know which two-way radio suits your situation best, so keep reading and we’ll help you decide.
If you already know your frequencies, the choice is pretty straightforward.
A VHF frequency might look like 152.2500. Land mobile commercial VHF radios have an operating range of 136-174 MHz, so in this case you need VHF equipment. But if your frequency is something like 452.2500, then you need UHF equipment. Land mobile commercial UHF radios have an operating range of 380-470, 400-470 or 450-512 MHz, depending on your needs.
Type your frequencyIf you’re starting a new talk group and haven’t picked your equipment yet, here are some tips for choosing between VHF and UHF
NOTE: Once you’ve decided between VHF and UHF, visit our antenna tutorial so you can make the right antenna choice!
VHF versus UHF
The most common two-way radio systems operate in the VHF and UHF parts of the radio spectrum. The VHF range is 136-174 MHz, while the UHF range covers 380-520 MHz. Both VHF and UHF radios are FM (frequency modulation), while CB (citizens band) radios are AM (amplitude modulation). FM radios are much clearer and have a more reliable range than AM radios such as CBs. UHF has a shorter wavelength, which makes it easier for the signal to find its way through walls and into buildings. VHF has a longer wavelength, which means it can transmit farther outdoors under ideal conditions. For most applications, lower radio frequencies are better for longer range. If you’ll be working mostly outdoors, a VHF radio is probably the best choice, especially with a base station radio and an external antenna. The basic rule of thumb for either VHF or UHF: the higher the antenna, the farther the radio can transmit and receive. One exception to VHF outdoors is heavily wooded or rugged terrain. There, a UHF radio may transmit better through the terrain (unless the VHF antenna is raised above it). If the radios are used mainly inside buildings, UHF is the best solution, since its shorter wavelength travels through buildings better.
VHF 136-174 MHz
longer wavelength
UHF 380-520 MHz
shorter wavelength
Basically it boils down to this:
- If you operate primarily outdoors, go with VHF. If you operate primarily indoors, go with UHF.
- If cost is a factor, typically VHF radios and antennas are a little less expensive than UHF radios.
Once you’ve picked VHF or UHF, visit our antenna tutorial so you can choose the right antenna.
Analog or Digital?
Another factor to consider is whether you need an analog or digital radio. For the most part, if you’re asking yourself “what is digital?”, odds are you don’t need it. Digital radios are used mainly in public service and, in some cases, the business world. They cost much more than traditional analog radios, but they offer some unique features the public service and business communities require. There is APCO P25 digital, which is the standard for all public service organizations. Then there is IDAS digital, which is unique to ICOM products and is a much more affordable alternative to P25 radios. Vertex also offers an alternative to P25 digital. In a nutshell, unless you know you need digital or P25 equipment, odds are you don’t. If you’re looking for basic two-way communications and don’t want to spend a small fortune, just stick with traditional analog radios. They make up most of what we sell on our website. If a radio is P25 or digital, its description says so.
Range
The useful direct range of a two-way radio system depends on radio propagation conditions. These are a function of frequency, antenna height and characteristics, atmospheric noise, reflection and refraction within the atmosphere, transmitter power, receiver sensitivity and the signal-to-noise ratio the modulation method requires. When a two-way radio system is engineered, the coverage of each base station is calculated along with an estimate of how reliable communication will be at that range. Two-way systems in the VHF and UHF bands, where many land mobile systems operate, rely on line-of-sight propagation for reliable coverage. The “shadowing” effect of tall buildings can block reception in places that would be within line-of-sight range out in open countryside. You can estimate the approximate line-of-sight distance to the radio horizon with this formula: horizon in kilometers = 3.569 times the square root of the antenna height in meters. Other factors also affect a two-way radio’s range, such as weather, the exact frequency used and obstructions.
Try it Drag the slider to raise the antenna and watch the reach grow.
Radio horizon 13.0 km
horizon in kilometers = 4.12 × √(antenna height in meters)
You can also install a repeater system, which relays the signal to increase the communication distance. Here’s an example of how a repeater works:
Note: Repeater operation is a basic feature built into every two-way radio we sell (except marine and aviation radios). To set up a repeater system, you’ll still need a repeater, a duplexer, an antenna and coaxial cable.
Marine or Aviation Radios?
If you need a radio for a boat or for aviation, you need a marine or aviation radio. Marine radios are FM and operate in the VHF spectrum between 155 and 160 MHz. Aviation radios operate between 118 and 136 MHz and use AM modulation. High Frequency Transceivers operate in all modes (FM, AM, SSB, CW, RTTY) below 30 MHz.
OK, that’s about as simple as we can make it. If you’re still unsure what you need or have any questions, feel free to email us.
Advanced Knowledge
Simplex
Simplex channel systems use a single channel to transmit and receive. This is typical of aircraft VHF AM and marine radios. Simplex systems are often legacy systems that have existed for years or decades. The architecture allows old radios to work with new ones in a single network. With all the ships or all the aircraft in the world, the large number of radios installed (the installed base) can take decades to upgrade. Simplex systems often use open architectures that let any radio meeting basic standards work with the entire system.
Advantage: as the simplest setup, it’s reliable, since only two radios are needed to communicate, without any other infrastructure.
Disadvantages: simplex offers the shortest range, because mobile units must be within range of each other. The available channel bandwidth limits the number of simultaneous conversations, since “dead” air time can’t easily be used for more communication.
Duplex
Duplex means two channels are used: one in each direction.
Duplex channel systems transmit and receive on different discrete channels. In these systems, equipment can’t communicate without some infrastructure, such as a repeater, base station or Talk-Through Base. The most common setup in the US is a repeater, where a base station re-transmits the audio it receives from mobile units. This lets mobiles and hand-helds talk to one another anywhere within reception range of the base station or repeater. The base or repeater station typically has a high antenna, which gives it greater range than a vehicle or hand-held transceiver.
Duplex systems can be divided into two types. Half-duplex refers to systems where you press a push-to-talk switch to communicate. Full duplex refers to systems like mobile phones that can receive and transmit at the same time.
Advantage: duplex channels usually allow repeater operation, which extends range, especially where hand-held radios are in use. In most cases that comes from more transmit power and a better, higher antenna location.
Disadvantage: if a radio can’t reach the repeater, it can’t communicate.
Hybrid simplex/duplex
Some systems use a mix of the two. Radios use duplex by default but can switch to simplex on the base station channel when they’re out of range. In the US, talking simplex on a duplex channel with a repeater is sometimes called talk-around, direct or car-to-car.
Types
Two-way radio systems can be classified in several ways depending on their attributes.
Conventional versus Trunked:
Conventional
Conventional radios operate on fixed RF channels. Radios with multiple channels operate on one channel at a time. The user picks the channel with a channel selector or buttons on the radio’s control panel.
In multi-channel systems, channels are used for separate purposes. A channel may be reserved for a specific function or for a geographic area. In a functional channel system, one channel may let City of Springfield road repair crews talk to the city’s road maintenance office. A second channel may let those crews talk with state highway department crews. In a geographic system, a taxi company may use one channel in the Boston, Massachusetts area and a second channel when its taxis are in Providence, Rhode Island. In marine radio, one channel is used as an emergency and calling channel. Stations make contact there and then move to a separate working channel to keep talking.
Motorola uses the term “mode” for channels on some conventional two-way radio models. A mode is a radio frequency channel plus all its channel-dependent options, such as selective calling.
Scanning in conventional radios
Some conventional radios scan more than one channel. That is, the receiver searches several channels for a valid transmission. A valid transmission may be a channel with any signal, or a channel combined with a specific CTCSS (or selective calling) code.
There is a wide variety of scan configurations, and they vary from one system to another. Some radios play the primary selected channel at full volume and the other channels in a scan list at reduced volume. This helps the user tell the primary channel from the others without looking at the radio’s control panel. An overview:
A scanning feature can be defined and preset: in scanning mode, a predetermined set of channels is scanned, and the radio user can’t change them.
Some radios offer user-selected scan, which lets the operator lock out preset channels or add channels to a scan list. The radio may go back to a default scan list each time it’s powered off, or it may permanently store the most recent changes.
In professional radios, scan features are programmable and have many options. Scan features can affect system latency. If the radio has a twenty-channel scan list and some channels have CTCSS, it can take several seconds to search the entire list. The radio must stop on each channel with a signal and check for a valid CTCSS before it resumes scanning. This can cause missed messages.
For this reason, in emergency applications, scan features are either not used or scan lists are kept intentionally short. Part of APCO Project 16 set standards for channel access times and delays caused by system overhead. Scan features can add to these delays. One study said delays longer than 0.4 seconds (400 milliseconds) are not recommended in emergency services. Zero delay between pressing push-to-talk and hearing the user’s voice in the radio’s speaker is an unattainable ideal.
Trunked
In a trunked radio system, the system logic automatically picks the physical radio frequency channel. A protocol defines the relationship between the radios and the radio backbone that supports them, and it lets channel assignments happen automatically.
Digital trunked systems may carry simultaneous conversations on one physical channel. In a digital trunked system, the system also manages time slots on that single physical channel. Carrying simultaneous conversations over a single channel is called multiplexing.
Instead of channels, radios are organized into groups. These may be called groups or talk groups, or divided into a hierarchy such as fleet and subfleet, or agency-fleet-subfleet. Think of them as virtual channels that appear and disappear as conversations happen.
Systems handle handshaking and connections between radios in one of two ways:
- A computer assigns channels over a dedicated control channel. The control channel sends a continual data stream. All radios in the system monitor the data stream until the computer tells them to join a conversation on an assigned channel.
- Electronics built into each radio use a protocol of tones or data to set up a conversation (scan-based).
If all physical channels are busy, some systems include a protocol to queue or stack pending requests until a channel becomes available.
Some trunked radios scan more than one talk group or agency-fleet-subfleet.
- 1) no squelch knob
- 2) no monitor button
- 3) a chirp
Visual clues that a radio may be trunked: 1) no squelch knob or adjustment, 2) no monitor button or switch and 3) a chirp (made infamous by Nextel) that tells you the channel is available and ready the moment you press push-to-talk.
Project 25 (APCO P25 Digital)
- 1988FCC inquiry, at the directive of the United States Congress
- October 1989APCO Project 25 is formed
- Since the 1990sPublic safety radios upgraded from analog to digital
APCONASTDNTIANCSNSADoDFPICCoast GuardNIST
Project 25 (P25 or APCO-25) is a suite of standards for digital radio communications. Federal, state/province and local public safety agencies in North America use it to communicate with other agencies and mutual aid response teams in emergencies. In this regard, P25 fills the same role as the European Terrestrial Trunked Radio (TETRA) protocol, although the two aren’t interoperable. Public safety radios have been upgraded from analog to digital since the 1990s, because of the limitations of analog transmission and the technological advances and expanded capabilities of digital radio.
Varying user protocols and different public safety radio spectrum made it hard for public safety agencies to achieve interoperability and widespread acceptance. However, lessons learned from disasters the United States faced in the past decade forced agencies to assess what they need during a disaster, when basic infrastructure has failed. To meet the growing demands of public safety digital radio communications, the United States Federal Communications Commission (FCC) began an inquiry in 1988 at the directive of the United States Congress. It gathered recommendations from users and manufacturers on improving the communication systems of the day. Based on those recommendations, APCO Project 25 was formed in October 1989 to find solutions that best serve the needs of public safety management. It was a coalition of the Association of Public-Safety Communications Officials-International (APCO), the National Association of State Telecommunications Directors (NASTD), the National Telecommunications and Information Administration (NTIA), the National Communications System (NCS), the National Security Agency (NSA) and the Department of Defense (DoD). A steering committee was also set up to decide the priorities and scope of P25’s technical development. It included representatives from those agencies, along with FPIC (Department of Homeland Security Federal Partnership for Interoperable Communication), the Coast Guard and the Department of Commerce’s National Institute of Standards and Technology (NIST) Office of Law Enforcement Standards.